<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>Untitled Document</title>
<style>
	#div1 {height: 200px;background: red;border: 1px solid black;}	
</style>
<script>
setInterval(function(){
	var oDiv = document.getElementById('div1');
	console.log(parseInt(oDiv.style.width));
	
	oDiv.style.width = parseInt( oDiv.style.width) -1 +'px';
},2000);	
</script>
</head>

<body>
<div id="div1" style="width: 200px; "></div>
</body>
</html>
